Many useful organic compounds contain hetero-atoms, e.g oxygen, sulfur, selenium, tellurium, nitrogen, phosphorus, arsenic, and antimony. Some of the simplest hetero-atom compounds are obtained by the insertion of a hetero-atom between the carbon and the hydrogen of hydrocarbon structures, e.g. oxygen inserted into one of the carbon hydrogen bonds of methane generates methanol, an alcohol. Such substitutions produce the rich and varied structural chemistry of carbon compounds. When the hetero-atom is not oxygen, e.g. sulfur, selenium, tellurium, nitrogen, phosphorus, arsenic, and antimony, lone pair electrons on the hetero-atom may participate in the formation of a dative bond to an oxygen atom creating a correlative oxidized product. In the case of an organic amine the correlative oxidized product is an amine N-oxide; phosphine oxides are similarly derived from phosphines. In the case of organic sulfur compounds, the two lone electron pairs resident on the sulfur atom of a thioether will participate in such an oxidation in a stepwise fashion forming first a sulfoxide and then a sulfone.
Frequently, the correlative oxidation product acquires new structural properties as a consequence of being oxidized. Tertiary amines of the formula RaRbRcN when oxidized to form the N-oxide, RaRbRcNO, exhibit the same type of stereochemistry as carbon compounds where when a given carbon atom in a structure is substituted by four different substitutents, optical isomers isomers are generated, i.e. enantiomeric isomers. While it is possible to surmise that non-symmetric tertiary amines may exist as optical isomers, the well known interconversion of structures through the reversible umbrellation of the nitrogen atom rapidly moots such a stereochemical question (the term non-symmetric is used herein in an inclusive fashion to encompass the more specific terms dissymetric and asymmetric). This is not the case for organic sulfur compounds, where the lone electron pair on sulfur is not labile in the fashion of nitrogen, it does not undergo interconversion between enantiomers. Thus the first oxidation of a non-symmetric thioether, e.g. RaRbS, leads to a sulfur oxide that exists in enantiomeric forms because the lone electron pair of the sulfur is stereochemically significant, i.e. it is not stereochemically labile in the sense that the molecule rapidly undergoes the umbrellation typical of the lone pair on the nitrogen atom of amines. Further oxidation of the sulfoxide to the sulfone destroys the optical isomerism because now the sulfur atom has two identical substitutents, the two oxygen atoms of the sulfone.
Many of the N-oxide compounds, the sulfoxides and the like have significant uses regardless of their stereochemistry. For example, tertiary N-oxides are excellent oxidation inhibitors in polymers. When thermoplastic polymers are processed at high temperatures such as those occurring in extrusion where the polymer must be molten, when one of the substitutents of the tertiary amine is a methyl group the tertiary N-oxide can under go an elimination reaction to produce a substituted hydroxylamine compound in situ that acts to prevent thermal degradation of the polymer.
In contrast, it has been found that many drugs exist in enantiomeric or mesomeric forms and that one optical isomer is pharmaceutically more active than other isomers. As long as some of the enantiomeric forms of these compounds are biologically inert they present only economic issues in the manufacture of the active ingredient. However, it has occasionally been found that while one enantiomeric isomer of a pharmaceutically active compound is beneficial the other, or others, may be detrimental. This leads to problems of stereochemical control in order to isolate the desired therapeutically active form of the molecule.
The conversion of amines in the synthesis of many drugs or to complex methods of purifying the active ingredients, phosphines, thioethers and the like to their correlative oxides by free radical oxidation such as the use of atmospheric oxygen being bubbled through a liquid reaction medium or using peroxide compounds, e.g. hydrogen peroxide or peracetic acid, creates just these sorts of problems, introducing a center of optical isomerism into the converted molecule, i.e. a chiral center, and creating enantiomers where one of the enantiomers is more pharmaceutically active than the other enantiomers.
Coordination complexes of transition metals may exist in a variety of isomeric forms broadly characterized as positional isomerism, i.e. the same set of atoms but a different set of bonds, or stereoisomerism, i.e. the same set of atoms and bonds but different symmetries. Broadly the types of positional isomerism in coordination complexes are:    1) linkage isomerism exists when the coordinating ligand may bond to the transition metal through more than one atom, example: complexes of ONO(NO2), when bonded through the nitrogen the complex is a nitro complex and when bonded through the oxygen the complex is a nitrito complex;    2) coordination isomerism exists when differing central metal ions can exchanged between differing sets of ligands, e.g. (Cr(NH3)6)(Co(CN)6) versus (Co(NH3)6)(Cr(CN)6);    3) ligand isomerism exists when the group bonding to the central metal ion can be substituted into different positions of the ligand molecule, e.g. (Co(1,2-diaminopropane)3)+3 versus. (Co(1,3-diaminopropane)3)+3;    4) solvation isomerism (or more specifically with H2O, hydration isomerism) exists when the solvent is coordinated versus being part of the crystal structure as in water of hydration, e.g. (Cr(H2O)6)Cl3 versus (Cr(H2O)5Cl)Cl2—H2O;    5) ionization isomerism exists when species are exchanged between the coordination sphere and simple ions, e.g. (Pt(NH3)4(OH)2)SO4 versus Pt(NH3)4SO4)(OH)2; and    6) polymerism isomerism exists when the molecular weights are different but the empirical weights are the same, e.g. (Pt(NH3)2Cl2)n versus (Pt(NH3)4)(PtCl4).
Stereoisomerism broadly exists in two major classes:    1) geometric isomerism exemplified by cis trans isomerism in square planar or octahedral complexes; and    2) optical isomerism.
Optical isomerism as it exists in transition metal complexes is similar to the optical isomerism observed in carbon compounds. A simplified view of optical isomerism in transition metal chemistry is when there is no center or plane of symmetry passing through the central metal ion of a transition metal compound or complex and the ion, compound or molecule may exist in two or more forms that possess an identical chemical formula and molecular weight but wherein the differing forms of the molecule can not be interconverted by rotation of substitutent groups around molecular bonds. Thus the molecules exist in so-called enantiomeric isomers or if two or more centers of asymmetry exist in the molecule, diastereomeric isomers. The terms dissymetric and asymmetric have acquired a particularly specific meaning in the field of stereochemistry; as used herein the term non-symmetric will be used herein to refer generally and inclusively to those chemical structures that would properly be more specifically described either as dissymetric or asymmetric. Non-symmetric complexes of transition metals have been found to be useful in the synthesis or preparation enantiomeric organic compounds, either by more or less standard chemical reactions or reactions that have been catalyzed either by homogeneous or heterogeneous catalysts.